If your dog is getting older and slower, you may be worried about how much weight she has gained. Or maybe you have a dog that once chowed down with gusto, but now seems to have lost interest in food. When a beloved pet ages, its eating habits and dietary needs can change. WebMD talked with two veterinarians to learn more about the challenges of feeding a senior dog.

At what age is a dog considered senior or geriatric?

“It really depends on the breed and body weight,” says Fred Metzger, DVM, Diplomate ABVP. “Large and giant breeds age faster than smaller dogs.” Metzger is the owner of Metzger Animal Hospital in State College, Pa. He is also an adjunct professor at Penn State University. He tells WebMD that in addition to breed making a difference, overweight dogs also age faster than lean dogs. “Just like humans,” he says.

As a rule of thumb, says Mark Nunez, DVM, dogs are considered older when they've reached half of their life expectancy. Nunez is president of the California Veterinary Medical Association and owner of a veterinary practice in Van Nuys, Calif. In general, “little dogs,” he says, “live to about 15 to 20 years of age, while bigger dogs live to about 12 to15 years.” Consequently, bigger dogs are considered older at around six years, and smaller dogs become older at around eight or nine.

Do senior dogs have special nutritional needs?

“Probably the most important thing for a geriatric dog is that their energy requirement gets lower,” Nunez says. With a slower metabolic rate, older dogs are more likely to become overweight or obese.

Many dog food companies now offer “senior” dog food formulations, which Nunez recommends as an “age-appropriate” diet for older pets because they're lower in calories.

If possible, owners should feed their pets foods that are suitable to their stage in life. However, some owners have more than one dog and would prefer to buy just one type of food. In that case, foods labeled “multi-stage” would be acceptable for puppies, adults, and seniors. “You make some compromises when you do the ‘multi-stage' diets,” Nunez says. “So they're my second choice. But some people just can't separate the foods. The puppy will get into the senior diet, and the senior dog will get into the puppy food.”

Metzger tells WebMD that when it comes to snacks, you should serve your senior dog healthy, low-fat, low-sodiumtreats. Although many dog owners think of bones and milk biscuits as snacks, there are alternatives. “Vegetables are great,” Metzger says. “Most dogs like carrots and apple slices.” But avoid grapes and raisins because they're harmful to dogs.

Older dogs also become dehydrated a lot faster than younger dogs, Nunez says. “The body's ability to maintain water balance is decreased as they get older,” he says. It's important to make sure that senior dogs have plenty of water.